Recently, in view of improving performances of engine, such as high power output and low fuel consumption, cylinder pressure of engine tends to be raised. Since engine bearing requires improvement of bearing performance under high surface pressure, lead-containing material has been widely used as a sliding bearing. However, usage of lead is restricted as it is an environmentally hazardous substance. So, lead-free high-performance bearing is demanded.
With respect to the bearing for high-load engine, usage of a silver material for overlay and liner has been examined. For example, in diesel engine for railway cars in other countries, silver alloy is sometimes used for piston pin bearing and turbo charger bearing. Moreover, silver-containing material and silver-plating material is sometimes used for not only overlay and liner but also sliding surface of the portion other than bearing, as a solid lubrication film (for example, refer to Patent documents 1 to 6).
In other countries, for diesel engine for railway car, many studies about lubricating oil which is applicable for the silver-containing material have been done (refer to Patent documents 7 to 25). For instance, Patent document 7 discloses a zinc-free lubricating oil containing: a hydrocarbon amine salt of dialkyl dithiophosphoric acid; a hydrocarbon amine salt of acidic alkyl phosphate; and a detergent, as a lubricating oil suitably used for high-load engine using silver-containing material, wherein the detergent is particularly preferably a phenate detergent. In this way, in other countries, to the devices using silver-containing material, zinc dialkyl dithiophosphate-free lubricating oil has been applied.
In these days in Japan, to the ball bearing used at the sliding portion of connecting-rod bearing of two-wheel vehicles, for the purpose of inhibiting abrasive wear, silver plating or copper plating having a thickness of more than a dozen micrometers has been given on the surface of the iron-based material as the base material. The silver plating and copper plating have excellent property in small fatigue phenomenon; however, its sulfidation corrosion is large, which is problematic. The actual corrosion is controlled by many elements thereby the mechanism is complex; but, it is known that the main cause of sulfidation corrosion is attributed to sulfur-containing compounds such as zinc dialkyl dithiophosphate contained in the engine oil. In general, so as to inhibit sulfidation corrosion, corrosion inhibitor such as benzotriazole is used. The corrosion inhibitor such as benzotriazole is useful for inhibiting sulfidation corrosion of copper; however, it has been discovered that it is not sufficiently effective for inhibiting corrosion of silver. Therefore, as a lubricating oil composition in contact with a silver member, a zinc dialkyl dithiophosphate-free lubricating oil composition is proposed (see Patent document 26).
Nevertheless, in view of anti-wear property and antioxidant characteristic, zinc dialkyl dithiophosphate is an essential component for lubricating oil for internal combustion. Therefore, development of lubricating oil composition which contains zinc dialkyl dithiophosphate and which can inhibit sulfidation corrosion of silver has been a big issue.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-195266    Patent Document 2: JP-A No. 2000-240657    Patent Document 3: JP-A No. 10-061727    Patent Document 4: JP-A No. 09-257045    Patent Document 5: JP-A No. 07-151148    Patent Document 6: JP-A No. 06-264110    Patent Document 7: JP-A No. 2007-023289    Patent Document 8: British Patent No. 1415964    Patent Document 9: Canadian Patent No. 810120    Patent Document 10: U.S. Pat. No. 2,959,546    Patent Document 11: U.S. Pat. No. 3,267,033    Patent Document 12: U.S. Pat. No. 3,649,373    Patent Document 13: U.S. Pat. No. 3,775,321    Patent Document 14: U.S. Pat. No. 4,169,799    Patent Document 15: U.S. Pat. No. 4,244,827    Patent Document 16: U.S. Pat. No. 4,278,553    Patent Document 17: U.S. Pat. No. 4,285,823    Patent Document 18: U.S. Pat. No. 4,575,431    Patent Document 19: U.S. Pat. No. 4,717,490    Patent Document 20: U.S. Pat. No. 4,764,296    Patent Document 21: U.S. Pat. No. 4,820,431    Patent Document 22: U.S. Pat. No. 5,244,591    Patent Document 23: U.S. Pat. No. 5,302,304    Patent Document 24: US Patent Application Laid-open (US-A) No. 2004/0259743    Patent Document 25: US-A No. 2005/0026791    Patent Document 26: JP-A No. 2002-294271